There are extensive published reports relating to the production of hydrophilic colloids by the aerobic propagation of the bacteria of the genus Xanthomonas in aqueous nutrient media. The earliest work in this field was done at The Northern Regional Research Laboratory of The United States Department of Agriculture at Peoria, Illinois and is described in U.S. Pat. No. 3,000,790. Modified fermentation processes are described in U.S. Pat. Nos. 3,020,206; 3,391,060; 3,427,226; 3,433,708; 3,271,267; 3,251,749; 3,281,329; 3,455,786; 3,565,763; 3,594,280; and 3,391,061.
The hydrophilic colloid (xanthan gum) produced by Xanthomonas campestris is a polysaccharide which contains mannose, glucose, glucuronic acid, O-acetyl radicals and acetal-linked pyruvic acid in the molar ratio of 2:2:1:1:0.5. The gum and its derivatives have found wide food and industrial applications. Of special interest is the increasing focus on the use of xanthan gum in displacement of oil from partially depleted reservoirs.
Typically, oil is recovered from underground reservoirs via a series of sequential operations. A new well will generally produce a limited amount of oil as a result of release of internal pressure in the well. As this pressure becomes depleted, it is necessary to pump further quantities of oil by mechanical means. These measures recover only about 25% of the total oil stored in the reservoir. A great deal of oil is still trapped within the pores of the formation. Further enhancement of recovery can then be effected by secondary recovery. In one method of recovery a waterflood is carried out by pumping water into a well or series of wells, displacing part of the trapped oil from the porous rock and collecting the displaced oil from surrounding wells. However, waterflooding still leaves about 55-60% of the available oil trapped in the formation. The explanation for this phenomenon is that the water has a very low viscosity compared to the crude oil and tends to follow the path of least resistance, fingering through the oil and leaving large pockets untouched. In addition, surface forces in the formation tend to bind the oil and prevent its displacement.
A number of processes have been developed in recent years to recover further quantities of oil from these reservoirs by the use of mobility control solutions which enhance oil displacement by increasing the viscosity or permeability of the displacing fluid. Of interest are those enhanced recovery processes employing polymer flooding with a polysaccharide or polyacrylamide to increase the viscosity of the displacing fluid. Variations of this process include the use of surfactants and co-surfactants to release the oil from the rock formation. Polyacrylamides have been found to suffer such deficiencies as viscosity loss in brines and severe shear sensitivity. Since, as was well documented in the prior art, xanthan gum is insensitive to salts (does not precipitate or lose viscosity under normal conditions), is shear stable, thermostable and viscosity stable over a wide pH range, xanthan gum is a good displacing agent. Moreover, the gum is poorly adsorbed on the elements of the porous rock formations and it gives viscosities useful in enhanced oil recovery (5 to 90 centipoise units at 1.32 sec..sup.-1 shear rate) at low concentrations (100 to 3000 ppm).
The use of solutions of xanthan gum or derivatives of xanthan gum for oil recovery is described in U.S. Pat. Nos. 3,243,000; 3,198,268; 3,532,166; 3,305,016; 3,251,417; 3,391,060; 3,319,715; 3,373,810, 3,434,542 and 3,729,460. It is suggested in U.S. Pat. No. 3,305,016 that aqueous solutions containing the heteropolysaccharide in sufficient quantity to increase the viscosity be employed as the thickening agent in preparing viscous waterflooding solutions. The polysaccharide may be prepared, separated, purified and then added. Alternatively, according to this reference, the entire culture, after adding a bactericide (e.g., formaldehyde) to kill the bacteria, may be added to the flood water.
U.S. Pat. No. 3,000,790 describes the culturing of a Xanthomonas bacterium in a well aerated medium containing commercial glucose, an organic source of nitrogen, dipotassium phosphate and appropriate trace elements. The source of organic nitrogen usually employed is distillers' solubles. The use of this organic nitrogen source contributes a substantial quantity of insolubles to the fermentation broth.
The processes described in U.S. Pat. Nos. 3,000,790, 3,391,060 and the other fermentation processes previously listed yield final fermentation broths that contain substantial amounts of insoluble matter, even when diluted with water, for injection into oil-bearing subterranean formations to impart the necessary and desired mobility control for oil displacement. The particulate matter and in certain cases the Xanthomonas cells in such whole broth would soon plug the oil-bearing formation at the site of injection and thus foul the well and prevent any further oil recovery. Furthermore, the same problem would be encountered with reconstituted xanthan gum precipitated and separated from these fermentation broths. The plugging tendencies of these fermentation broths can be obviated by filtration through diatomaceous earth leaf filters to remove the Xanthomonas cells and particulate and colloidal matter. However, such additional filtering steps are expensive and add significantly to the overall cost factors for enhanced oil recovery.
U.S. Pat. No. 3,853,771 approaches the plugging problem of whole fermentation broths by claiming a process for dissolving or dispersing cellular microorganisms which comprises contacting said materials with an aqueous solution containing at least one surfactant effective for dispersing outer wall layers of microorganism cells, at least one chelating agent for dispersing the inner wall layers of microorganism cells, and at least one alkali metal hydroxide effective for enhancing said dispersing actions.
U.S. Pat. No. 4,010,071 describes a process for clarifying fermentation broths and other aqueous suspensions containing a dissolved xanthan gum and suspended solids resulting from the fermentation by treatment with a minor amount of a protease enzyme. The injectivity of aqueous solutions containing xanthan gum so clarified is improved in oil well flooding operations over solutions not so treated. However, this treatment does not overcome plugging problems due to the presence of insoluble inorganic or non-proteinaceous organic materials present in the fermentation medium or produced during the course of the fermentation.
The object of the process of U.S. Pat. No. 3,391,060 is to recover a polysaccharide product of substantial purity without the use of extensive separation procedures. A high quality, high viscosity, light colored, high purity xanthan gum is recovered from a Xanthomonas fermentation broth. According to this patent, recovery is simplified and elaborate purification procedures are obviated by the replacement of the organic source of nitrogen in the broth with an inorganic source of nitrogen (i.e., ammonium nitrate). However, the fermentation process described therein is not economical because of lengthy reaction times required and low yields of biopolymer obtained.
The method of improving the permeability of mobility control solutions by the addition of certain hydroxy substituted carboxylic acids such as malic, tartaric, citric, gluconic, lactic and salicylic is described in U.S. Pat. No. 2,867,279.